Balanced modulator system



April 3o, 1946.4

H. TOOMlM BALANCED MODULATOR SYSTEM Filed spt .l 11, 1945 #5x5/m4 750mm i lNVENToR ATTORN EY either of the .of the following L mentes Apr." 3o, 194e' 2,399,586 BALANcsn MonULA'roa sYs'rsM ner-snel frcnmi'm, Press Wireless, of Delaware Great Neck, N.Y., assigner to Inc., Chicago, Ill.,

a corporation c Application September 11, 1943, Serial No. 501,928

. 6 Claims.

This invention relates to modulating systems and 'more especially to electron tube modulators of the balanced type.

A principal object of the invention is to provide an improved double-balanced modulator.

Another object is to provide an double-balancedmodulator of the type wherein two input frequencies are intermodulated to produce in the output their sum and difference frequencies without any appreciable presence of input frequencies. A

Another object is to provide an double-balanced modulator wherein frequencies are intermodulated to produce in the output their sum vand difference frequencies without any appreciable presence of the harmonics of the signal input frequencies.

Another object isl to provide an improved in-` verting network especially useful in secrecy transmission andthe like, whereby a normal intelligence frequency e. g., 200 C. l?. S. can be converted into a higher frequency signal e. g., 2000 C. P. S.

A feature of the inventionl relates to a stable and relatively cheap double-balanced electron tube modulator.

Another feature relates to a double-balanced electron tube modulater employing a carrier con trol frequency in the form of sharply squared carrier waves in the nature of an electronic switch control.

Other features and advantages not specifically enumerated will be apparent after a consideration detailed descriptions and the improved appended claims.

In the drawing embodiment of the invention, the numeral l rep- 'resents any well-known source of intelligence signals of varying frequency which, 'for example, are to be converted into a higher frequency. The

--nu'meral 2 represents any wel -known source of constant frequency square waves which are balanced with respect to ground. The waves from source 2 will. for convenience, be referred to as carrier waves. Preferably the amplitude of the signals from the amplitude of the signals from source 2. The sources I andy 2, instead of being of the form shown, may consists of any well-known push-pull balanced source of signals.'

Associated with source I is a pair of grid-controlled tubes 3, 4, whose control grids I. 6. are connected respectively across the input transformer 1 provided withra grounded midpoint lsecondary as is well-known in push-pull systems. The plates or anodes 8, 9, are connected in like which represents one preferred source I is very much smaller than positive terminal of the D. C. plate supply` source improved two input quency.

`a linear transfer characteristic so ear degenerative Vof the contact I'1, and by the 179-1115) phase through an output load resistor I0 to the Il. The signals to be utilized the terminals 2B, 29, across resistor I0. The cathodes I2 and I3 of tubes 3 and 4 connected in parallel through respective bias resistors I4, IB, which in turn are connected to a common resistor I6, which is returned to ground through adjustable contact I1.

The source 2 is likewise connected in balanced push-pull relation across the control grids I8, I9, of a. pair of grid-controlled tubes 20, 2l. The plates or anodes 22,23 of these tubes, are connected in like phase with the positive terminal of source II. The cathodes 24, 25, are connected by conductors 2B, 21, directly to the corresponding cathodes I2. I3.

The system can -be balanced by an adjustment suitable balanced adjustment of the input circuits and to grids I8 and I9. For this latter purpose, a grounded midpoint transformer may be employed, or if desired a potentiometerI resistance may be connected across each pair of grids and with its sliding contact connected" to ground. Other well-known methods of balancing the input circuits from lthe sources I and 2 may be utilized.

I have found that by using a source 2 of carrier waves of sharply squared wave shape. the phase of the intelligence signals from source I can be reversed at the carrier frequency with a minimum production of harmonics of the intelligence fre- This is so because the system provides far as intelliare tapped ofi at gence signals are concerned. suppression of the frequencies from sources I and 2, as well as substantial suppression of the harmonics of the intelligence frequency. y

v- The manner of operation of the system is along the follcwing lines. As the grid I8, for example, becomes instantaneously positive for the duration of each positive square wave from source 2, tube 20 becomes conductive. 20 flows through bias resistor of which biasses grid l5 to substantial plate current. cutoff. 'Tube 3 therefore remains substantiaily non-conductive during each positive onehalf cycle impressed on grid IB from source 2. However, during this half cycle, grid I9 maintains tube 2| non-conductive with the result that I4, the voltage drop tube 4 is conductiveand remains conducting for each positive' one-half cycle impressed on grid IB. Tube 4 under these conditions, acts as alinampliiier of unity gain if RIII=RI4=RI5. When the carrier cycle from to grids 5 and l6, t

There is completey The plate current of tube y between the plate source 2 reverses and grid I8 becomes positive, tube 4 becomes non-conductive and tube I becomes conductive. Tube 3 then acts as a linear degenerative amplifier of unity gain, amplifying the signal in opposite phase to that amplified by tube 4. The resultant output of tubes 3 and Lremains substantially constant, but by reason of the balanced out-of-phase inputs to tubes 3, 4 and 20, 2|, and by reason of the signal in-phase output load lil, the output at terminals 28, 29, contains large sum and difference of the -frequencies from sources I and 2. Thus, if source l is delivering at any given instant, a 200 C. P. S. intelligence signal, and source 2 delivers an 1800 C. P. S. square wave carrier, the output at terminals 28 and 29 will contain 2000 C. P. S. and 1600 C. P. S. If desired, the terminals 28 and 29 may be connected to a suitable high-pass filter to pass the sum frequencies only. i

While one particular embodiment has been clisclosed herein, various changes and modifications may be made without departing from the spiritl and scope of the invention.

What I claim is: Y

1. In a double-balanced modulator, a first pair of grid-controlled electron tubes with their input circuits connected in balanced push-pull relation to a source of intelligence waves, a second pair of grid-controlled tubes with their input circuits connected in balanced push-pullv relation to a source of square carrier waves, a separate grid bias resistor for each tube of the first pair, said grid bias resistors being connected in divided balanced relation in the plate-to-cathode circuits of the corresponding tubes of the second pair, and a balancing potentiometer connected between said bias resistors and the midpoint of the balanced system to cause the transfer first pair of tubes to be substantially linear so far as said intelligence waves are concerned.

2. In a double-balanced modulator, a pair of grid-controlled tubes having their input circuits characteristic of the connected in balanced push-pull relation to a source of signal waves, an electronic switch comprising a pair of grid-controlled electron tubes having their input electrodes connected in balanced push-pull relation to a source of carrier frequency square-topped waves, and connections pair of tubes and the grid bias circuits of the first mentioned pair of tubes for alternately biasing the first mentioned pair of tubes to plate current cut-off in synchronlsm with said squarecircuits of the second mentioned' said voltages in divided topped waves. the output electrodes of said first pair of tubes being connected in like phase to a load circuit whereby the signal wave `frequency vand the square-wave frequency are deleted in said output circuit and the said output circuit passes substantially the sum and diii'erence frequencies between said signal frequency .and said squarewave frequency.

3. In a. double balanced modulator system, a first pair of gridlcontrolled electron tubes having their input electrodes connected in divided balanced relation to a source of waves representing intelligence to be conveyed, a second pair of gridcontrolled electron tubes having their input electrodes connected in divided balanced relation to a separate source of sharply squared carrier waves of constant frequency, means for applying the plate currents of said second pair of tubes to de velop corresponding voltages and for applying balanced relation to the cathodes of the first pair of tubes to cause plate current cut-off bias to be applied to said first pair of tubes alternately in accordance with the reversal of phase of the carrier waves, and a com mon output lad circuit connected in like phase to the plates of said first pair of tubes whereby the signals appearing in said output circuit consist substantially only of the sum and difference frequencies between the waves from said two sources.

4.. A double balanced modulator system according to claim 3 in which the means for'developing and applying said voltages includes separate cathode resistors for the first pair of tubes so that the transfer characteristic between the input and output of each of said first pair of tubes is substantially linear so far as said intelligence signals are concerned.

5. A double balanced modulator system according to claim 3 in which the means for developing and applying said voltages includes individual cathode resistors in the cathode return circuits of said first pair of tubes to cause each tube of said first pair to act as a lineal` degenerative amplifier.

6. A double balanced modulator system according to claim 3 in which the means for developing and applying said voltages includes individual cathode load resistors forvthe rst pair of tubes,

the 'said first pair of tubes to act alternately as linear degenerative amplifiers of unity gain.

HERSHEL TOOMIM. 

